The present invention is directed to a method for generating a random number for the encoded transmission of data in a data exchange system having processor chip cards, using a secret cipher, a ciphering algorithm allocated to a random number generator, and a variable starting value that is logically combined with a most recently used and intermediately stored random number.
Protection of data is an ever more important aspect regarding modern data processing and communication systems. The degree of quality of a system with respect to adequate data protection is critically dependent upon the extent to which one succeeds in providing possible access to the system only for authorized persons and, conversely, of keeping unauthorized persons locked out with absolute reliability. A simple, if not absolutely reliable possibility for checking the access authorization to a system is, for example, the use of passwords that are only known to the authorized user and that can be changed by the user with arbitrary frequency. Since there is always the risk that passwords can be discovered or overheard by unauthorized persons, additional security measures are indispensable. One of these measures, for example, is the encoding and decoding of the transmitted information, a measure that can be realized in data processing systems by using a chip card. A drawback regarding the increasing use of the chip card in communications systems, however, is the additional security risk due to the fact that chip cards can be relatively easily lost. Care must therefore be exercised to a great degree to ensure that a lost chip card is protected against potential misuse. The chip card is therefore designed such that the data stored in a protected chip card can only be accessed when an identifier stored in the chip card, for example, a personal identification number (PIN) is input in advance by the user.
A further security barrier to the system can be established by requiring an authentication of the chip card. This authentication prevents an arbitrary person or user of the system from accessing secret information in the system by pretending to be authorized. A critical prerequisite for the authentication is a personal feature of the user that cannot be copied. This feature of the user that cannot be copied is determined by using a secret cipher for the coding and decoding that is known to the two pertinent general components or partners of the system, i.e., first, the chip card and, second, the system itself. This feature is known only to these two partners. The security can be additionally enhanced by use of an arbitrary number that is transmitted by the chip card to the system, the arbitrary number being generated in the chip card upon use of the secret cipher. It would also be conceivable to produce these random numbers in a program-oriented fashion. In the opinion of security experts, however, random numbers generated in such fashion are not sufficiently random and, thus, are not ultimately reliable.
It is known that to increase the "randomness" of a random number, the generation of the random number can be made dependent on a variable starting value or, even better, this variable starting value can be modified with a random number that was previously generated.